Distributed coherent communications are Radio Frequency (RF) communications where coherent transmissions are made from a transmit antenna array, and/or RF transmissions are received by a synchronized receive antenna array. Each of the antenna arrays may be formed by an array of nodes, with each of the nodes having one or more of the antennas. Some or all of the nodes of either transmit (Tx) node array or receive (Rx) node array may be ad hoc nodes (as is described below). The “coherent” property of the coherent communications refers to synchronization of the nodes, so that (1) each of the nodes in the transmit array can transmit synchronously, and/or (2) each of the nodes of the receive array are synchronized and the received signals may be combined using a common time reference, with resulting transmit and/or receive array gain. Distributed coherent communications may offer significant link budget gains and increased performance over those available with single-antenna-to-single-antenna communications. Compared to the use of single antenna transceivers, the use of multiple antennas in wireless networks offers the promise of increased data rates, reach distance, battery life, anti-jam capabilities, spectral reuse, as well as reduced latency. Distributed coherence can be leveraged into transmit beamforming (with, e.g., N2-fold increase in power for N transmit antennas), receive beamforming (with, e.g., M-fold increase in power for M receive antennas), and simultaneous transmit/receive beamforming gains (with a theoretically-possible N2M-fold increase in power for N transmit to M receive antennas).
Distributed communication and networking approaches, however, may suffer from one or more of a number of disadvantages, including these:
1. Some systems may improve established point-to-point links, but cannot create links which would otherwise have not existed;
2. Some systems rely on complex weights and pre-coding matrices calculations derived from link measurements, which may not always be available or easily obtainable;
3. Some systems employ long latency feedback techniques, which may inject extra delays and may not converge in time sufficiently short for operation in fast-changing dynamic and otherwise challenging environments;
4. Some systems rely on channel training in the forward and return directions to estimate weights and pre-coding matrices (i.e., “closed loop” or “explicit” approach), with associated complexity and delays;
5. Some systems require brute force, system-wide, long-term and short-term synchronization of carrier frequency, modulation, data, and time.
At a given array, collaborative beamforming weights may be discovered using a “sounding” signal, an opportunistic or an intentional signal emitted by the object of the beamforming, as discussed in more detail below. The underlying assumption of such collaborative beamforming communications is that a single target node is capable of emitting a sounding signal with sufficient power to overcome link losses to reach the beamforming array, which can then retrodirect the signal back to the target node as a many-to-one (N:1) beamformer. In cases of one-to-many (1:M) receive beamforming where a single node emits a signal and an array of nodes on the receiving side uses beamforming weights to receive the signal, the underlying assumption is that the emitting node transmits with sufficient power for the signal to traverse the channel and be detected by the array of nodes on the receiving side.
It is not always the case, however, that a single transmitting node can transmit with power sufficient to be detected on the other side of the link; in other words, a single node may not be able to “close” the link on its own, particularly where the transmitted signal is a sounding signal and initially is not beamformed. For a many-to-many (N:M) array-to-array beamforming, where a pair of arrays communicate with one another, simultaneous transmit and receive gains can be used to enhance the link budget. Link budget gains may be applied, for example, to link improvement or reduction in transmit power. Table 1 below is a summary of possible benefits from simultaneous transmit and receive beamforming gains in exemplary systems:
TABLE 1LPI/LPD/BatterySpectral(PA)Data RateReachAnti-jamLatencyReuseLifeLink Improvement↑Log2(N2 · M)↑N · {square root over (M)}↑N2 · M↓Hops↓N—(Full TX Power)Stealth (1/N2 TX↑Log2(M)↑{square root over (M)}↑M—↑N↑N2Power)
Such gains may be achieved when transmit and receive beamforming weights are derived using optimal algorithms and applied at the Tx and Rx arrays. The correct channel information between the array members on each side of the link and a mechanism for the nodes to transmit/receive a corrected and weighted signal at each of the array nodes are needed, so that beamforming is achieved to within an accuracy required by the system. If the array nodes are distributed (i.e., untethered physically separated radios such as ad hoc nodes described below), each array may implement a distributed algorithm across the nodes of the array, enabling the array to operate in a coherent manner, providing frequency, and phase/time synchronization/alignment of the clocks and oscillators of the different nodes of the array. Certain array synchronization/alignment methods for local synchronization of a distributed set of array nodes are described in some of the related and commonly-owned patent documents listed below.
The problem of initially detecting a signal such as a sounding signal without a priori knowledge of the target, the environment, and the beamforming weights, however, remains.
There is a need for techniques for improving radio frequency communications, and in particular for techniques to improve beamforming in circumstances where sounding transmissions from a single node of a target array may not be detectable by the transmitting array. There is also a need for techniques for establishing and maintaining RF communications between arrays with improved beamforming weights, in both static and dynamically-changing environments, and in both Line-of-Sight (LoS) and Non-Line-of-Sight situations (NLoS).